I'll vote against "In a mirror, darkly pt. 1" because honestly, what's so great about the mirror episodes, apart from them having the TOS nostalgia bonus? They may be good, but not among the best of the show imo. Besides, featuring characters from an alternative universe exclusively, this story has next to no meaning for main ENT storyline.

I will almost certainly be in the minority here, but my vote will go to "TWILIGHT".

This was an excellent episode by any standard, but my only gripe is it ends with a reset, which the nature of the episode makes it impossible for it not to be one. Because of that, you essentially know the ending before the theme begins.

For me, the other three episodes edge out due to the built-in reset. I will say, however, that if "TWILIGHT" does end up being the winner, I will not be sad. ANY of these remaining well deserve that honor.

Going with the mirror ep here. Because I’m not a fan of mirror episodes. And as much as I likes mirror Reed, Phlox and Hoshi, Mirror Archer and Mayweather-and the silly Gorn ..just made it laughable to me.
Plus, Azati Prime is one of my fave Trek episodes ever.

I'll give a point to Twilight. It is great, but the truth is, it's just easier to be great with a reset ending. The other episodes are all a higher degree of difficulty to pull off, from their basic premises.